[PATCH v2 5/5] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER

[Caleb Sander Mateos <csander@purestorage.com>]

io_ring_ctx's mutex uring_lock can be quite expensive in high-IOPS
workloads. Even when only one thread pinned to a single CPU is accessing
the io_ring_ctx, the atomic CASes required to lock and unlock the mutex
are very hot instructions. The mutex's primary purpose is to prevent
concurrent io_uring system calls on the same io_ring_ctx. However, there
is already a flag IORING_SETUP_SINGLE_ISSUER that promises only one
task will make io_uring_enter() and io_uring_register() system calls on
the io_ring_ctx once it's enabled.
So if the io_ring_ctx is setup with IORING_SETUP_SINGLE_ISSUER, skip the
uring_lock mutex_lock() and mutex_unlock() for the io_uring_enter()
submission as well as for io_handle_tw_list(). io_uring_enter()
submission calls __io_uring_add_tctx_node_from_submit() to verify the
current task matches submitter_task for IORING_SETUP_SINGLE_ISSUER. And
task work can only be scheduled on tasks that submit io_uring requests,
so io_handle_tw_list() will also only be called on submitter_task.
There is a goto from the io_uring_enter() submission to the middle of
the IOPOLL block which assumed the uring_lock would already be held.
This is no longer the case for IORING_SETUP_SINGLE_ISSUER, so goto the
preceding mutex_lock() in that case.
It may be possible to avoid taking uring_lock in other places too for
IORING_SETUP_SINGLE_ISSUER, but these two cover the primary hot paths.
The uring_lock in io_uring_register() is necessary at least before the
io_uring is enabled because submitter_task isn't set yet. uring_lock is
also used to synchronize IOPOLL on submitting tasks with io_uring worker
tasks, so it's still needed there. But in principle, it should be
possible to remove the mutex entirely for IORING_SETUP_SINGLE_ISSUER by
running any code needing exclusive access to the io_ring_ctx in task
work context on submitter_task.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Tested-by: syzbot@syzkaller.appspotmail.com
---
 io_uring/io_uring.c |  6 +++++-
 io_uring/io_uring.h | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 69e1175256bb..b743644a3fac 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3534,12 +3534,15 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 		if (ret != to_submit) {
 			io_ring_ctx_unlock(ctx);
 			goto out;
 		}
 		if (flags & IORING_ENTER_GETEVENTS) {
-			if (ctx->syscall_iopoll)
+			if (ctx->syscall_iopoll) {
+				if (ctx->flags & IORING_SETUP_SINGLE_ISSUER)
+					goto iopoll;
 				goto iopoll_locked;
+			}
 			/*
 			 * Ignore errors, we'll soon call io_cqring_wait() and
 			 * it should handle ownership problems if any.
 			 */
 			if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
@@ -3556,10 +3559,11 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			 * We disallow the app entering submit/complete with
 			 * polling, but we still need to lock the ring to
 			 * prevent racing with polled issue that got punted to
 			 * a workqueue.
 			 */
+iopoll:
 			mutex_lock(&ctx->uring_lock);
 iopoll_locked:
 			ret2 = io_validate_ext_arg(ctx, flags, argp, argsz);
 			if (likely(!ret2))
 				ret2 = io_iopoll_check(ctx, min_complete);
diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index a0580a1bf6b5..7296b12b0897 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -121,20 +121,34 @@ bool io_match_task_safe(struct io_kiocb *head, struct io_uring_task *tctx,
 
 void io_activate_pollwq(struct io_ring_ctx *ctx);
 
 static inline void io_ring_ctx_lock(struct io_ring_ctx *ctx)
 {
+	if (ctx->flags & IORING_SETUP_SINGLE_ISSUER) {
+		WARN_ON_ONCE(current != ctx->submitter_task);
+		return;
+	}
+
 	mutex_lock(&ctx->uring_lock);
 }
 
 static inline void io_ring_ctx_unlock(struct io_ring_ctx *ctx)
 {
+	if (ctx->flags & IORING_SETUP_SINGLE_ISSUER) {
+		WARN_ON_ONCE(current != ctx->submitter_task);
+		return;
+	}
+
 	mutex_unlock(&ctx->uring_lock);
 }
 
 static inline void io_ring_ctx_assert_locked(const struct io_ring_ctx *ctx)
 {
+	if (ctx->flags & IORING_SETUP_SINGLE_ISSUER &&
+	    current == ctx->submitter_task)
+		return;
+
 	lockdep_assert_held(&ctx->uring_lock);
 }
 
 static inline void io_lockdep_assert_cq_locked(struct io_ring_ctx *ctx)
 {
-- 
2.45.2